Passive stagnation control for solar collectors

ABSTRACT

A method for controlling stagnation in a solar collector, comprises: providing an solar energy absorbing substrate and a first layer; providing a second layer disposed between the first layer and the solar energy absorbing substrate; coupling an actuator to the solar energy absorbing substrate; and expanding the actuator when the solar collector is exposed to a stagnation temperature to form a gap between the first layer and the second layer.

BACKGROUND

Solar collectors can be efficient and cost-effective sources of hotwater for domestic and commercial hot water heating as well as for spaceheating. Polymeric solar collectors commonly are constructed from atransparent polymer glazing sheet (e.g., polycarbonate multi-wallsheet), a black polymeric absorber with extruded water channels (e.g.,polysulfone or polyphenylene ether blend multi-wall sheet), aninsulating backing, and water manifolds and frame pieces. Since theabsorber is insulated from both the front and back, temperatures muchhigher than ambient can be attained. Solar collectors are commonlydesigned to produce water as hot as 70 degrees Celsius (° C.) to 80° C.

There can be periods when the collector is exposed to the sun, but wateror other heat transfer fluid is not flowing through the absorber causingthe solar collector to overheat. These conditions are termed “stagnationconditions.” Absorber temperatures in excess of 130° C. or even 140° C.are possible during these stagnation conditions. During stagnationconditions, the heat deflection temperature of the plastic componentscan be exceeded, resulting in irreversible buckling, thermal expansionbeyond design limits, and/or other thermally-induced effects that canlead to failure of the unit. Using only polymers capable of withstandingsuch temperatures greatly increases the cost of the collector. Thus,control of stagnation temperature is an important factor for efficient,cost-effective plastic solar collectors.

Accordingly, there is a need for a solar collector that can help controlstagnation temperatures without the need for expensive materials orelaborate controls to provide an efficient, cost-effective polymericsolar collector with a lifetime of greater than 20 years.

SUMMARY

Disclosed, in various embodiments, are solar collectors, and methods forusing the same.

A solar collector, comprises: a solar energy absorbing substrate; afirst layer and a second layer above the solar energy absorbingsubstrate, each of the first layer and the second layer includingrespective surface features facing one another and defining aninterface, the surface features configured to transmit a selectedportion of solar energy across the interface when in alignment with oneanother, the transmission being greater than or equal to 80% measuredaccording to ISO 9060:1990; and an actuator coupled to the solar energyabsorbing substrate in thermal communication with the solar energyabsorbing substrate, the actuator mechanically coupled with at least oneof the first layer and the second layer; wherein, the actuator isconfigured to receive thermal energy from the solar energy absorbingsubstrate and move the first layer and the second layer to move therespective surface features out of alignment with light transmissionthrough the first layer being less than or equal to 10% measuredaccording to ISO 9060:1990.

A method for controlling stagnation in a solar collector, comprising:providing an solar energy absorbing substrate and a first layer;providing a second layer disposed between the first layer and the solarenergy absorbing substrate; coupling an actuator to the solar energyabsorbing substrate; and expanding the actuator when the solar collectoris exposed to a stagnation temperature to form a gap between the firstlayer and the second layer.

These and other features and characteristics are more particularlydescribed below.

BRIEF DESCRIPTION OF THE DRAWINGS

The following is a brief description of the drawings wherein likeelements are numbered alike and which are presented for the purposes ofillustrating the exemplary embodiments disclosed herein and not for thepurposes of limiting the same.

FIG. 1 is a cross-sectional side view of a solar collector.

FIG. 2 is a cross-sectional side view of the solar collector of FIG. 1at a stagnation temperature.

FIG. 3 is a cross-sectional side view of a solar collector at astagnation temperature.

FIG. 4 is a cross-sectional side view of a solar collector at astagnation temperature with a pulley system.

FIG. 5 is a cross-sectional view of various geometric configurations foran interface between components of the solar collector.

FIG. 6 is a cross-sectional side view of a solar thermal collectorincluding an intermediate layer.

DETAILED DESCRIPTION

Disclosed herein are solar collectors including a light absorber (alsoreferred to herein as “absorber” and “solar energy absorbing substrate”)and multiple layers that allow transmission of light when the layers arein contact and prevent transmission of light when the layers areseparated. By preventing transmission of light, the solar collectorsdescribed herein can advantageously lower the temperature of the lightabsorber and can assist in preventing stagnation conditions. Separationof the layers can occur by a variety of methods. For example, the lightabsorber and/or actuator can expand at a stagnation temperature. Theexpansion of the light absorber and/or actuator can then mechanicallyseparate the layers, reducing the temperature within the solar collectorto below the stagnation temperature. In addition, by utilizing thermalexpansion of the light absorber and/or actuator, a simple low-costsolution can be obtained without the need for complex assemblies, movingparts, or expensive control systems.

For instance, mechanical louvers could be made to open at elevatedtemperatures and thereby open the module to release heat, but thisintroduces moving parts and control systems, increases complexity andcost, and provides additional failure mechanisms. Some concepts usingthermo-responsive materials for thermal control can rely on, forexample, a phase separation process or an abrupt phase transition, bystrongly differing temperature dependencies of the refractive indices ofdomains and matrix, and/or can rely on a change in their visible opticalproperties to cause scattering of light and attenuate the amount oflight that can reach an solar energy absorbing substrate (e.g., certainhydrogels and polymer blends with critical temperatures for miscibility,liquid crystals, etc.). However, none of these systems are practical orcost-effective for a polymeric solar collector since they involvecomponents that are fluid or involve difficult to tailor and expensivechemical material components.

The solar collectors disclosed herein can include multiple layers (e.g.,a multilayer sheet) and a solar energy absorbing substrate (e.g., alight absorbing layer), wherein the multiple layers can be located abovesolar energy absorbing substrate layer. The multiple layers cangenerally be transparent (e.g., have greater than or equal to 60 to 80%solar radiation (λ between 300 nanometers (nm) and 2500 nm)transmission) when the layers are joined together, and less than 20%transmission (i.e., opaque) when the layers are separated. The solarenergy absorbing substrate can be black, meaning that it will not haveany transmission. The solar energy absorbing substrate can absorbincoming light and transfer the energy to a circulating fluid, such asair, water, ethylene glycol, etc. The solar energy absorbing substratecan be made of any material with the desired thermal and hydrolyticstability. Examples include polysulfones, modified poly(phenyleneoxides), polyetheretherketone (PEEK), polyimide, and combinationscomprising at least one of the foregoing. The multiple layers caninclude layers made from a single material or each layer can includegreater than one material with the different materials having the samelight transmission and/or refractive index (e.g., +/−20%).

The various layers can be designed to include a complimentary geometricconfiguration to one another at an interface between the multiplelayers. For example, the multiple layers can include a complimentarylamellar-shaped construction, triangular-shaped construction,pyramidal-shaped construction, cylindrical-shaped construction,conical-shaped construction, cubical-shaped construction,trapezoidal-shaped construction, sinusoidal-shaped construction, sawtooth-shaped construction, abs(sin)-shaped construction, cycloid-shapedconstruction, fiber shaped construction, saw tooth-shaped construction,pyramidal-shaped construction, and combinations thereof at the interfacebetween the layers.

The separation of the multiple layers at temperatures at or above thestagnation temperature in combination with the complimentary geometricconfiguration can assist in reflection of the incoming light, whichduring stagnation periods, can reduce the temperature extremesexperienced by the solar collector, thereby resulting in a lowerlikelihood of failure (e.g., buckling, warping, thermal expansion, etc.)of the other components of the solar collector. Accordingly, the solarcollectors disclosed herein can provide protection to the various othercomponents of the solar collector (i.e., panels) (e.g., solar panels)against failure or damage due to exposure to temperatures above the heatdeflection temperature of the components.

An air gap can be present between the multiple layers and the solarenergy absorbing substrate with an actuator formed in the air gap. Theactuator can, when exposed to temperatures at or above stagnationconditions, expand to push the first layer away from the second layer,creating a gap therebetween. The air gap located between the first layerand the second layer can assist in blocking incoming light from reachingthe solar energy absorbing substrate, thereby allowing the solar energyabsorbing substrate to return to its pre-stagnation temperature. Oncethe pre-stagnation temperature has been reached, the actuator cancontract, once again bringing the first layer and the second layer backinto contact with one another. The air gap can create a refractive index(RI) mismatch between the polymeric material having a RI of, e.g., 1.5and air having a RI of 1.0. The air gap can be at least as thick as thewavelength of the incoming light. The air gap can be 0.01 to 5millimeters (mm). The air gap can be 0.05 to 3 mm. The air gap can be0.1 to 2.0 mm. The air gap can be 0.25 to 1.5 mm.

For example, a solar collector can include a solar energy absorbingsubstrate; a first layer and a second layer above the solar energyabsorbing substrate. The first layer and the second layer can includerespective surface features facing one another and can define aninterface. The surface features can be configured to transmit a selectedportion of solar energy across the interface when in alignment with oneanother. The transmission can be greater than or equal to 80% asmeasured according to ISO 9060:1990. The solar collector can include anactuator coupled to the solar energy absorbing substrate in thermalcommunication with the solar energy absorbing substrate, where theactuator can be mechanically coupled with at least one of the firstlayer and the second layer. The actuator can be configured to receivethermal energy from the solar energy absorbing substrate and move thefirst layer and the second layer to move the respective surface featuresout of alignment. When the surface features are out of alignment withone another, light transmission through the first layer can be less thanor equal to 10% as measured according to ISO 9060:1990.

A more complete understanding of the components, processes, andapparatuses disclosed herein can be obtained by reference to theaccompanying drawings. These figures (also referred to herein as “FIG.”)are merely schematic representations based on convenience and the easeof demonstrating the present disclosure, and are, therefore, notintended to indicate relative size and dimensions of the devices orcomponents thereof and/or to define or limit the scope of the exemplaryembodiments. Although specific terms are used in the followingdescription for the sake of clarity, these terms are intended to referonly to the particular structure of the embodiments selected forillustration in the drawings, and are not intended to define or limitthe scope of the disclosure. In the drawings and the followingdescription below, it is to be understood that like numeric designationsrefer to components of like function.

As illustrated in FIG. 1, the solar collector 1, can include first layer10, second layer 20, which, when in contact with one another can includea high light transmission (e.g., greater than or equal to 80% lighttransmittance). In other words, when not separated from one another,first layer 10 and second layer 20 can act as a single optical body witha high light transmittance as described herein (e.g., optical coupling).When separated, the optical coupling between first layer 10 and secondlayer 20 is broken, and the first layer 10 can reflect light away fromsolar energy absorbing substrate 30 through internal reflection. Percentlight transmission can be determined according to ISO 9060:1990 using apyranometer (e.g., a thermopile pyranometer).

For example, when first layer 10 and second layer 20 are in contact(e.g., abutting with no gap therebetween), first layer 10 and secondlayer 20 can have greater than or equal to 80% transmittance measuredaccording to ISO 9060:1990. When in contact (e.g., abutting with no gaptherebetween), first layer 10 and second layer 20 can have greater thanor equal to 85% transmittance measured according to ISO 9060:1990. Whenin contact (e.g., abutting with no gap therebetween), first layer 10 andsecond layer 20 can have greater than or equal to 90% transmittancemeasured according to ISO 9060:1990.

As shown in FIG. 1, first layer 10 and second layer 20 can include acomplementary saw tooth construction at the interface between thelayers. FIG. 5 illustrates some of the complimentary geometricconfigurations that first layer 10 and second layer 20 can include atthe interface between the layers. For example, the complimentarygeometric configurations can include trapezoidal-shaped configurations120. The complimentary geometric configurations can include sawtooth-shaped configurations 130. The complimentary geometricconfigurations can include sinusoidal-shaped configurations 140. Thecomplimentary geometric configurations can include lamellar-shapedconfigurations 150. The complimentary geometric configurations caninclude triangular-shaped configurations 160. The complimentarygeometric configurations can include abs(sin)-shaped configurations 170.The complimentary geometric configurations can include cycloid-shapedconfigurations 180.

As shown in FIG. 1, light beam 5 can pass through first layer 10 andsecond layer 20 and air gap 40 to provide energy to solar energyabsorbing substrate 30. Actuator 32 can be coupled to solar energyabsorbing substrate 30. In addition, multiple actuators (i.e., greaterthan one) can be included. For example, an array of actuators can beutilized where the array of actuators can be dispersed across a length,1, of the solar collector. As used herein, coupled refers to joiningthrough mechanical joining (e.g., welding, and/or fastener(s)) and/orchemical joining (e.g., adhesive, glue, etc.). Actuator 32 can be madefrom the same material as that of solar energy absorbing substrate 30,thereby having the same coefficient of thermal expansion as solar energyabsorbing substrate 30. Actuator 32 can also be made from a differentmaterial than that of solar energy absorbing substrate 30 where thematerial has a coefficient of thermal expansion that is greater than orequal to that of solar energy absorbing substrate 30. Actuator 32 andsolar energy absorbing substrate 30 can optionally include fillers thatincrease the coefficient of thermal expansion, as set forth in U.S. Pat.No. 8,552,101, which is incorporated herein by reference in itsentirety. For example, the fillers can include an intrinsic thermalconductivity greater than or equal to 50 W/mK. Suitable fillers include:AlN (Aluminum nitride), BN (Boron nitride), MgSiN₂ (Magnesium siliconnitride), SiC (Silicon carbide), Ceramic-coated graphite, Graphite,Expanded graphite, Graphene, Carbon fiber, Carbon nanotube (CNT), orGraphitized carbon black, or combinations thereof.

As illustrated in FIGS. 1 and 2, at or above a stagnation temperature,actuator 30 can expand to contact first layer 10 at contact area 12 toseparate first layer 10 and second layer 20 and create gap 15therebetween first layer 10 and second layer 20. Gap 15 can be greaterthan or equal to 2500 nm. Gap 15 can be greater than or equal to 0.1 mm.Gap 15 can be greater than or equal to 0.5 mm. Gap 15 can be 0.05 to 5mm. Gap 15 can be greater than or equal to 0.075 to 3 mm. Gap 15 can be0.1 to 2 mm. Gap 15 can be 0.25 to 1.5 mm. As shown in FIG. 6, anintermediate layer 25 can be positioned at the interface of first layer10 and second layer 20. Intermediate layer 25 can function as anadhesive layer to assist in holding the first layer 10 and second layer20 together during conditions below the stagnation temperature.Intermediate layer 25 can have a refractive index similar to first layer10 and second layer 20. For example, the refractive index ofintermediate layer 25 can be within 10% of the refractive index of firstlayer 10 and second layer 20. The refractive index of intermediate layer25 can be within 5% of the refractive index of first layer 10 and secondlayer 20. The refractive index of intermediate layer 25 can be within 4%of the refractive index of first layer 10 and second layer 20. Therefractive index of intermediate layer 25 can be within 3% of therefractive index of first layer 10 and second layer 20. The refractiveindex of intermediate layer 25 can be within 2% of the refractive indexof first layer 10 and second layer 20. In addition, intermediate layer25 can include a light transmission identical to the light transmissionof first layer 10 and second layer 20. Intermediate layer 25 can have ageometric configuration complementary to both first layer 10 and secondlayer 20 such that when positioned together (e.g., below stagnationtemperature) there is no gap between first layer 10, intermediate layer25, and second layer 20.

When actuator 30 pushes first layer 10 and second layer 20 apart at astagnation temperature of the solar collector 1, intermediate layer 25can remain attached to either first layer 10 or second layer 20.Depending on the materials of the solar collector, during stagnation,the solar energy absorbing substrate can reach temperatures of greaterthan or equal to 110° C. The solar energy absorbing substrate can reachtemperatures of greater than or equal to 120° C. during stagnation. Thesolar energy absorbing substrate can reach temperatures of greater thanor equal to 130° C. during stagnation. The solar energy absorbingsubstrate can reach temperatures of greater than or equal to 140° C.during stagnation. The solar energy absorbing substrate can reachtemperatures of greater than or equal to 150° C. during stagnation. Thesolar energy absorbing substrate can reach temperatures of greater thanor equal to 160° C. during stagnation.

As shown in FIG. 2, when gap 15 is present between first layer 10 andsecond layer 20, light beam 5 reflects within first layer 10 such that atotal internal reflection is observed, (e.g., no light passes through tosolar energy absorbing substrate 30), allowing solar energy absorbingsubstrate 30 to cool and return to a non-stagnation temperature. Gap 15can act as a stopper to prevent light from reaching the solar energyabsorbing substrate 30 and thus, can allow the solar energy absorbingsubstrate to return to non-stagnation temperature. In the alternative,when the layers 10, 20 are separated by gap 15 at stagnationtemperature, less than or equal 20% of light can be transmitted to solarenergy absorbing substrate 30, which can also allow for a reduction inthe temperature of solar energy absorbing substrate 30 to apre-stagnation state. When the layers 10, 20 are separated by gap 15,less than 15% of light can be transmitted to solar energy absorbingsubstrate 30. When the layers 10, 20 are separated by gap 15, less than10% of light can be transmitted to solar energy absorbing substrate 30.When the layers 10, 20 are separated by gap 15, less than 5% of lightcan be transmitted to solar energy absorbing substrate 30. When thelayers 10, 20 are separated by gap 15, less than 2.5% of light can betransmitted to solar energy absorbing substrate 30. When the layers 10,20 are separated by gap 15, less than 1.0% of light can be transmittedto solar energy absorbing substrate 30. As solar energy absorbingsubstrate 30 and actuator 32 cool and contract, first layer 10 can bebrought back into contact with second layer 20 (or intermediate layer 25if intermediate layer 25 remains joined to second layer 20), which canallow light beam 5 to transmit to solar energy absorbing substrate 30(as shown in FIG. 1).

Alternative methods of creating gap 15 are shown in FIGS. 3 and 4. Forexample, FIG. 3 illustrates a mechanism that can include first wedge 50coupled to first layer 10 and second wedge 55 coupled to solar energyabsorbing substrate 30. As solar energy absorbing substrate 30 reaches astagnation temperature and expands laterally, second wedge 55 can movelaterally with respect to first wedge 50 illustrated by arrow 52 in FIG.3, which can force first wedge 50 vertically along a slope of secondwedge 55. Thus, first wedge 50 can force first layer 10 to separate fromsecond layer 20, thereby creating gap 15. As previously discussedherein, gap 15 can prevent transmittance of light beam 5, which canallow solar energy absorbing substrate 30 to cool and contract. Thecontraction of solar energy absorbing substrate 30 can allow secondwedge 55 to move back to its pre-stagnation position, which can allowfirst wedge 50 to slide down the slope of second wedge 55. As firstwedge 55 assumes its pre-stagnation position, first layer 10 can bebrought back into contact with second layer 20 (or intermediate layer 25if intermediate layer 25 remains joined to second layer 20), which canallow light beam 5 to transmit to solar energy absorbing substrate 30(as shown in FIG. 1).

FIG. 4 illustrates another actuator mechanism including a pulley system60 that can be coupled to second layer 20 at connection 61, and solarenergy absorbing substrate 30 at connection 62. As solar energyabsorbing substrate 30 reaches a stagnation temperature, it can expandlaterally. When this occurs, the position of connection 62 can be movedin the direction of arrow 54 in FIG. 4. Thus, pulley system 60 can drawsecond layer 20 vertically downward and away from first layer 10 therebycreating gap 15. As previously discussed herein, gap 15 can preventtransmittance of light beam 5, which can allow solar energy absorbingsubstrate 30 to cool and contract. The contraction of solar energyabsorbing substrate 30 can allow connection 62 to return to itspre-stagnation position, which can allow for second layer 20 to reunitewith first layer 10 (or intermediate layer 25 if intermediate layer 25remains joined to second layer 20), which can allow light beam 5 to onceagain transmit light to solar energy absorbing substrate 30 (as shown inFIG. 1).

Possible polymeric materials that can be employed for the first layer10, second layer 20, and solar energy absorbing substrate 30 can includeany transparent homopolymer, copolymer, or blend thereof. It can bedesirable for the polymeric materials to have optical transparency andstability toward light and heat. Examples of desirable polymers includepolyesters, polycarbonates, polystyrene, poly(methyl methacrylate)(PMMA), poly(ethyl methacrylate), poly(styrene-co-methyl methacrylate),poly(styrene-co-acrylonitrile) (SAN), poly(methylmethacrylate-co-styrene-co-acrylonitrile) (MMASAN), and other copolymersof styrene, acrylonitrile, various (meth)acrylic acids, and various(meth)acrylates, as well as combinations comprising at least one of theforegoing. The first layer 10, second layer 20, and solar energyabsorbing substrate 30 can be made from the same or different materialor any combination therebetween.

Possible polymeric materials that can be employed for the intermediatelayer 25 can include silicone based materials including a curablesilicone (such as a silicone thermoset elastomer (TSE), a UV curablesilicone, thermoplastic polyolefin (TPO), thermoplastic polyurethane(TPU), ethylene tetrafluoroethylene (ETFE), polyvinylidene fluoride(PVDF), ethylene vinyl acetate (EVA), or a room temperature vulcanize(RTV) silicone.

The refractive index of the polymeric material used for the first layer10, second layer 20 and/or solar energy absorbing substrate 30 can be0.5 to 2.5. The refractive index of the polymeric material used for thefirst layer 10, second layer 20 and/or solar energy absorbing substrate30 can be 1.0 to 2.0. The refractive index of the polymeric materialused for the first layer 10, second layer 20 and/or solar energyabsorbing substrate 30 can be 1.25 to 1.75. The polymeric material usedfor the first layer 10, second layer 20, and/or solar energy absorbingsubstrate 30 can have a total forward transmission of greater than orequal to 80%. The polymeric material can have a total forwardtransmission of greater than or equal to 85%. The refractive index ofthe polymeric material can have a total forward transmission of greaterthan or equal to 90%. The light transmission of the polymeric materialcan be 60% to 100%. The light transmission of the polymeric material canbe 65% to 90%. The light transmission of the polymeric material can be75% to 85%. Forward transmission refers to all light emanating from thenon-irradiated surface of the article, i.e., all light that is notreflected, absorbed, or transmitting over the edges. Forwardtransmission includes both direct transmission along the normal line aswell as any light scattered off-normal (haze). Measurement of totalforward transmission (or total reflection) is usually accomplished withthe use of a spectrometer equipped with an integrating sphere. Thepolymeric material can also include various additives ordinarilyincorporated into polymer compositions of this type, with the provisothat the additive(s) are selected so as to not significantly adverselyaffect the desired properties of the layers, in particular, the abilityto transmit or reflect incoming light. Examples of additives that can beincluded in the matrix polymer or the thermo-responsive layer includeoptical effects fillers, impact modifiers, fillers, reinforcing agents,antioxidants, heat stabilizers, light stabilizers, ultraviolet (UV)light stabilizers, plasticizers, lubricants, mold release agents,antistatic agents, colorants (such as carbon black and organic dyes),surface effect additives, radiation stabilizers (e.g., infraredabsorbing), gamma stabilizers, flame retardants, and anti-drip agents. Acombination of additives can be used, for example a combination of aheat stabilizer, mold release agent, and ultraviolet light stabilizer.In general, the additives are used in the amounts generally known to beeffective. Each of these additives can be present in amounts of 0.0001to 10 weight percent (wt. %), based on the total weight of the polymericmaterial.

For example, plasticizing agents can be used to adjust the refractiveindex and additives such as antioxidants and light stabilizers can alsobe present in the polymeric material. Plasticizers for inclusion in thepolymeric material can include benzoate esters of polyols such aspenterythritol tetrabenzoate, aliphatic esters, and aryl esters ofphosphates such as resorcinol bis(diphenyl phosphate), as well ascombinations comprising at least one of the foregoing.

The polymeric material can further optionally include a flame retardant.Flame retardants include organic and/or inorganic materials. Organiccompounds include, for example, phosphorus, sulfonates, and/orhalogenated materials (e.g., comprising bromine chlorine, and so forth,such as brominated polycarbonate). Non-brominated and non-chlorinatedphosphorus-containing flame retardant additives can be preferred incertain applications for regulatory reasons, for example organicphosphates and organic compounds containing phosphorus-nitrogen bonds.

Inorganic flame retardants include, for example, C₁₋₁₆ alkyl sulfonatesalts such as potassium perfluorobutane sulfonate (Rimar salt),potassium perfluorooctane sulfonate, tetraethyl ammonium perfluorohexanesulfonate, and potassium diphenylsulfone sulfonate (e.g., KSS); saltssuch as Na₂CO₃, K₂CO₃, MgCO₃, CaCO₃, and BaCO₃, or fluoro-anioncomplexes such as Li₃AlF₆, BaSiF₆, KBF₄, K₃AlF₆, KAlF₄, K₂SiF₆, and/orNa₃AlF₆. When present, inorganic flame retardant salts are present inamounts of 0.01 to 10 parts by weight, more specifically 0.02 to 1 partsby weight, based on 100 parts by weight of the polymeric material.

Light stabilizers and/or ultraviolet light (UV) absorbing stabilizerscan also be used. Exemplary UV light absorbing stabilizers includehydroxybenzophenones; hydroxybenzotriazoles; hydroxyphenyl triazines(e.g., 2-hydroxyphenyl triazines); cyanoacrylates; oxanilides;benzoxazinones; dibenzoylresorcinols;2-(2H-benzotriazol-2-yl)-4-(1,1,3,3-tetramethylbutyl)-phenol (CYAS ORB™5411); 2-hydroxy-4-n-octyloxybenzophenone (CYAS ORB™ 531);2-[4,6-bis(2,4-dimethylphenyl)-1,3,5-triazin-2-yl]-5-(octyloxy)-phenol(CYASORB™ 1164); 2-[4,6-diphenyl-1.3.5-triazin-2-yl]-5-(hexyloxy)-phenol(Tinuvin 1577), 2,2′-(1,4-phenylene)bis(4H-3,1-benzoxazin-4-one)(CYASORB™ UV-3638);1,3-bis[(2-cyano-3,3-diphenylacryloyl)oxy]-2,2-bis[[(2-cyano-3,3-diphenylacryloyl)oxy]methyl]propane(UVINUL™ 3030); 2,2′-(1,4-phenylene) bis(4H-3,1-benzoxazin-4-one);1,3-bis[(2-cyano-3,3-diphenylacryloyl)oxy]-2,2-bis[[(2-cyano-3,3-diphenylacryloyl)oxy]methyl]propane;4,6-dibenzoylresorcinol, nano-size inorganic materials such as titaniumoxide, cerium oxide, and zinc oxide, all with a particle size of lessthan or equal to 100 nanometers, or combinations comprising at least oneof the foregoing UV light absorbing stabilizers. UV light absorbingstabilizers are used in amounts of 0.01 to 5 parts by weight, based on100 parts by weight of the total composition, excluding any filler.

Anti-drip agents can also be used in the polymeric material, for examplea fibril forming fluoropolymer such as polytetrafluoroethylene (PTFE).The anti-drip agent can be encapsulated by a rigid copolymer, forexample styreneacrylonitrile copolymer (SAN). PTFE encapsulated in SANis known as TSAN. An exemplary TSAN includes 50 wt. % PTFE and 50 wt. %SAN, based on the total weight of the encapsulated fluoropolymer. TheSAN can include, for example, 75 wt. % styrene and 25 wt. %acrylonitrile based on the total weight of the copolymer. Anti-dripagents can be used in amounts of 0.1 to 10 parts by weight, based on 100parts by weight of the total composition of the particular layer,excluding any filler.

The first layer 10, second layer 20, and intermediate layer 25 can befabricated by any means including solvent casting, melt casting,extrusion, blow molding, injection molding or co-extrusion onto asubstrate.

The thickness of the layers (first layer 10, second layer 20, andintermediate layer 25) of the solar collector can vary depending uponthe thickness of the individual components of solar collector. Forexample, the thickness of the layers can be 25 μm to 2,500 μm. Thethickness of the layers can be 100 μm to 1,250 μm. The thickness of thelayers can be 250 μm to 1,000 μm. The thickness of the solar energyabsorbing substrate can be 1 mm to 55 mm. The thickness of the solarenergy absorbing substrate can be 2 mm to 35 mm. The thickness of thesolar energy absorbing substrate can be 2 mm to 25 mm. The thickness ofthe solar energy absorbing substrate can be 3 mm to 15 mm.

The thermo-responsive assemblies can likewise be used in any applicationwhere, for example, it is desirable to regulate temperature based onlight reflection (such as in solar panels, in photovoltaic applications,and in greenhouse applications). The layers of the solar collector(i.e., first layer 10, second layer 20, and/or intermediate layer 25)can be applied to a window (such as a vehicle window and a buildingwindow, for example, a greenhouse window, an office window, and a housewindow). The window can be glass and/or polymeric.

The solar collectors as described herein are further illustrated by thefollowing non-limiting examples.

It is to be understood that the materials and methods are not limited tothose disclosed herein and used in the examples. One skilled in the artwill readily be able to select suitable polymeric materials andvariations of methods to separate the layers to reflect light or reducetransmission to the solar energy absorbing substrate.

Set forth below are some embodiments of solar collectors and methods ofmaking the collectors as disclosed herein.

Embodiment 1: A solar collector, comprising: a solar energy absorbingsubstrate; a first layer and a second layer above the solar energyabsorbing substrate, each of the first layer and the second layerincluding respective surface features facing one another and defining aninterface, the surface features configured to transmit a selectedportion of solar energy across the interface when in alignment with oneanother, the transmission being greater than or equal to 80% measuredaccording to ISO 9060:1990; and an actuator coupled to the solar energyabsorbing substrate in thermal communication with the solar energyabsorbing substrate, the actuator mechanically coupled with at least oneof the first layer and the second layer; wherein, the actuator isconfigured to receive thermal energy from the solar energy absorbingsubstrate and move the first layer and the second layer to move therespective surface features out of alignment with light transmissionthrough the first layer being less than or equal to 10% measuredaccording to ISO 9060:1990.

Embodiment 2: The solar collector of Embodiment 1, wherein therespective surface features comprise complementary geometricconfigurations at the interface between the first layer and secondlayer.

Embodiment 3: The solar collector of any of Embodiments 1 or 2, whereineach geometric configuration includes at least one of trapezoidal,saw-tooth, sinusoidal, lamellar, triangular, abs(sin), cycloid, andpyramidal geometric configurations.

Embodiment 4: The solar collector of any of Embodiments 1-3, furthercomprising an intermediate layer located between the first layer andsecond layer.

Embodiment 5: The solar collector of Embodiment 4, wherein theintermediate layer comprises a material having a light transmissionequal to the light transmission of the first layer.

Embodiment 6: The solar collector of any of Embodiments 1-5, wherein theactuator and the solar energy absorbing substrate have identicalcoefficients of thermal expansion.

Embodiment 7: The solar collector any of Embodiments 1-6, wherein theactuator has a coefficient of thermal expansion greater than thecoefficient of thermal expansion of the solar energy absorbingsubstrate.

Embodiment 8: The solar collector of any of Embodiments 1-7, wherein thelight transmission through the first layer and the second layer is lessthan or equal to 5% measured according to ISO 9060:1990 at thestagnation temperature.

Embodiment 9: The solar collector of any of Embodiments 1-8, wherein thefirst layer and the second layer comprise a polymeric material selectedfrom polyester, polycarbonate, polystyrene, poly(methyl methacrylate),poly(styrene-co-methyl methacrylate), poly(styrene-co-acrylonitrile),poly(methyl methacrylate-co-styrene-co-acrylonitrile), and combinationscomprising at least one of the foregoing.

Embodiment 10: The solar collector of any of Embodiments 1-9, furthercomprising an air gap between the second layer and the solar energyabsorbing substrate.

Embodiment 11: The solar collector of Embodiment 10, wherein theactuator is located in the air gap.

Embodiment 12: A method for controlling stagnation in a solar collector,comprising: providing a solar energy absorbing substrate and a firstlayer; providing a second layer disposed between the first layer and thesolar energy absorbing substrate; coupling an actuator to the solarenergy absorbing substrate; and expanding the actuator when the solarcollector is exposed to a stagnation temperature to form a gap betweenthe first layer and the second layer.

Embodiment 13: The method of Embodiment 12, wherein each of the firstlayer and the second layer include respective surface features facingone another and defining an interface, the surface features configuredto transmit a selected portion of solar energy across the interface whenin alignment with one another, the transmission is greater than or equalto 80% measured according to ISO 9060:1990 and wherein when the surfacefeatures of the first layer and the second layer are moved out ofalignment, light transmission through the first layer is less than orequal to 10% measured according to ISO 9060:1990.

Embodiment 14: The method of any of Embodiments 12-13, wherein therespective surface features of the first layer and second layer comprisecomplementary geometric configurations at the interface between thefirst layer and second layer.

Embodiment 15: The method of any of Embodiments 12-14, wherein theactuator and the solar energy absorbing substrate comprise identicalcoefficients of thermal expansion.

Embodiment 16: The method of any of Embodiments 12-15, wherein the lighttransmission through the first layer and the second layer, when thesurface features are in direct contact with one another, is greater thanor equal to 80% transmission measured according to ISO 9060:1990.

Embodiment 17: The method of any of Embodiments 12-16, wherein the lighttransmission through the first layer and the second layer at thestagnation temperature is less than or equal to 5% transparency measuredaccording to ISO 9060:1990.

Embodiment 18: The method of any of Embodiments 12-17, wherein the firstlayer and the second layer comprise a polymeric material selected frompolyester, polycarbonate, polystyrene, poly(methyl methacrylate),poly(styrene-co-methyl methacrylate), poly(styrene-co-acrylonitrile),poly(methyl methacrylate-co-styrene-co-acrylonitrile), and combinationscomprising at least one of the foregoing.

Embodiment 19: The method of any of Embodiments 12-18, furthercomprising providing an air gap between the second layer and the solarenergy absorbing substrate.

Embodiment 20: The method of Embodiment 19, further comprising locatingthe actuator in the air gap.

All ranges disclosed herein are inclusive of the endpoints, and theendpoints are independently combinable with each other (e.g., ranges of“up to 25 wt. %, or, more specifically, 5 wt. % to 20 wt. %,” isinclusive of the endpoints and all intermediate values of the ranges of“5 wt. % to 25 wt. %,” etc.). “Combination” is inclusive of blends,mixtures, alloys, reaction products, and the like. Furthermore, theterms “first,” “second,” and the like, herein do not denote any order,quantity, or importance, but rather are used to determine one elementfrom another. The terms “a” and “an” and “the” herein do not denote alimitation of quantity, and are to be construed to cover both thesingular and the plural, unless otherwise indicated herein or clearlycontradicted by context. The suffix “(s)” as used herein is intended toinclude both the singular and the plural of the term that it modifies,thereby including one or more of that term (e.g., the film(s) includesone or more films). Reference throughout the specification to “oneembodiment,” “another embodiment”, “an embodiment,” and so forth, meansthat a particular element (e.g., feature, structure, and/orcharacteristic) described in connection with the embodiment is includedin at least one embodiment described herein, and may or may not bepresent in other embodiments. In addition, it is to be understood thatthe described elements may be combined in any suitable manner in thevarious embodiments.

All cited patents, patent applications, and other references areincorporated herein by reference in their entirety. However, if a termin the present application contradicts or conflicts with a term in theincorporated reference, the term from the present application takesprecedence over the conflicting term from the incorporated reference.

While particular embodiments have been described, alternatives,modifications, variations, improvements, and substantial equivalentsthat are or may be presently unforeseen may arise to applicants orothers skilled in the art. Accordingly, the appended claims as filed andas they may be amended are intended to embrace all such alternatives,modifications variations, improvements, and substantial equivalents.

1. A solar collector, comprising: a solar energy absorbing substrate; afirst layer and a second layer above the solar energy absorbingsubstrate, each of the first layer and the second layer includingrespective surface features facing one another and defining aninterface, the surface features configured to transmit a selectedportion of solar energy across the interface when in alignment with oneanother, the transmission being greater than or equal to 80% measuredaccording to ISO 9060:1990; and an actuator coupled to the solar energyabsorbing substrate in thermal communication with the solar energyabsorbing substrate, the actuator mechanically coupled with at least oneof the first layer and the second layer; wherein, the actuator isconfigured to receive thermal energy from the solar energy absorbingsubstrate and move the first layer and the second layer to move therespective surface features out of alignment with light transmissionthrough the first layer being less than or equal to 10% measuredaccording to ISO 9060:1990.
 2. The solar collector of claim 1, whereinthe respective surface features comprise complementary geometricconfigurations at the interface between the first layer and secondlayer.
 3. The solar collector of claim 1, wherein each geometricconfiguration includes at least one of trapezoidal, saw-tooth,sinusoidal, lamellar, triangular, abs(sin), cycloid, and pyramidalgeometric configurations.
 4. The solar collector of claim 1, furthercomprising an intermediate layer located between the first layer andsecond layer.
 5. The solar collector of claim 4, wherein theintermediate layer comprises a material having a light transmissionequal to the light transmission of the first layer.
 6. The solarcollector of claim 1, wherein the actuator and the solar energyabsorbing substrate have identical coefficients of thermal expansion. 7.The solar collector of claim 1, wherein the actuator has a coefficientof thermal expansion greater than the coefficient of thermal expansionof the solar energy absorbing substrate.
 8. The solar collector of claim1, wherein the light transmission through the first layer and the secondlayer is less than or equal to 5% measured according to ISO 9060:1990 atthe stagnation temperature.
 9. The solar collector of claim 1, whereinthe first layer and the second layer comprise a polymeric materialselected from polyester, polycarbonate, polystyrene, poly(methylmethacrylate), poly(styrene-co-methyl methacrylate),poly(styrene-co-acrylonitrile), poly(methylmethacrylate-co-styrene-co-acrylonitrile), and combinations comprisingat least one of the foregoing.
 10. The solar collector of claim 1,further comprising an air gap between the second layer and the solarenergy absorbing substrate.
 11. The solar collector of claim 10, whereinthe actuator is located in the air gap.
 12. A method for controllingstagnation in a solar collector, comprising: providing an solar energyabsorbing substrate and a first layer; providing a second layer disposedbetween the first layer and the solar energy absorbing substrate;coupling an actuator to the solar energy absorbing substrate; andexpanding the actuator when the solar collector is exposed to astagnation temperature to form a gap between the first layer and thesecond layer.
 13. The method of claim 12, wherein each of the firstlayer and the second layer include respective surface features facingone another and defining an interface, the surface features configuredto transmit a selected portion of solar energy across the interface whenin alignment with one another, the transmission is greater than or equalto 80% measured according to ISO 9060:1990 and wherein when the surfacefeatures of the first layer and the second layer are moved out ofalignment, light transmission through the first layer is less than orequal to 10% measured according to ISO 9060:1990.
 14. The method ofclaim 12, wherein the respective surface features of the first layer andsecond layer comprise complementary geometric configurations at theinterface between the first layer and second layer.
 15. The method ofclaim 12, wherein the actuator and the solar energy absorbing substratecomprise identical coefficients of thermal expansion.
 16. The method ofclaim 12, wherein the light transmission through the first layer and thesecond layer, when the surface features are in direct contact with oneanother, is greater than or equal to 80% transmission measured accordingto ISO 9060:1990.
 17. The method of claim 12, wherein the lighttransmission through the first layer and the second layer at thestagnation temperature is less than or equal to 5% transparency measuredaccording to ISO 9060:1990.
 18. The method of claim 12, wherein thefirst layer and the second layer comprise a polymeric material selectedfrom polyester, polycarbonate, polystyrene, poly(methyl methacrylate),poly(styrene-co-methyl methacrylate), poly(styrene-co-acrylonitrile),poly(methyl methacrylate-co-styrene-co-acrylonitrile), and combinationscomprising at least one of the foregoing.
 19. The method of claim 12,further comprising providing an air gap between the second layer and thesolar energy absorbing substrate.
 20. The method of claim 19, furthercomprising locating the actuator in the air gap.